The tri-substituted (2,5-dihydrophenyl)phosphonium salts are latent catalysts for promoting the reaction between vicinal epoxides and phenols and/or carboxylic acids (or anhydrides). Pre-catalyzed epoxy resins are easily prepared using such catalysts. This utility and a full description of the salts is set forth in our co-pending application Ser. No. 481,599 filed June 21, 1974, the disclosure of which is incorporated herein by reference. This application is directed to a method of preparing one of the classes of compounds set forth in Ser. No. 481,599 which is represented by formula I ##STR1## wherein R.sub.1 -R.sub.3 are hydrocarbyl or inertly-substituted hydrocarbyl radicals, each of which independently has from 1 to about 20 carbon atoms, and A.sup..crclbar. is a compatible neutralizing anion.
Such compounds were previously prepared by reacting the appropriate tri-substituted phosphine with p-benzoquinone in benzene solution. This method was described by Ramirez et al., J. Am. Chem. Soc., Vol. 78, 5614 (1956). In this technique, the phosphobetaine was first obtained as a solid precipitate which was isolated and subsequently converted to the corresponding phosphonium salt by reaction with an aqueous protic acid or a methanol/water solution of the acid.